"I never understand why people spend ages trying out fifty different distros if they could get just what they want if they spent half that time actually installing/adapting one distribution with a reasonably broad package base."
Because if you have to do it more than once, or, say onto 100 desktops in a work environment, the time spent searching for a distro will pay off. Besides that, some people want stuff to work the first time - and every subsequent time - without having to worry "oh I need to install this on another computer, but I can't remember all the steps I took" in order to get to real actual work.
If you just want the same programs on your laptop and desktop, use something like dpkg --get-selections and dpkg --set-selections (for APT, I’m sure RPMland has an analogue). If you also want the same configuration, copy over your homedir or the dotfiles therein and be happy.
Futhermore, testing n distros is not really my definition of ‘work the first time’, nor can you expect that what worked a year ago still works two releases later.
Your point about 100 desktops in a work environment also strikes me as slightly odd, given that I would consider using Puppet, Chef or possibly NFS mounts (depending on the exact setup) rather than configuring/installing 100 desktops individually.
As far as same programs, I would still have to load them the first time and know what to look for. So in addition to stuff I need for work (e.g. LAMP stack), I would have to Google around (more than once) for stuff that I know comes with other distros (example of slight annoyance: vim vs. vim-tiny: thanks, Ubuntu). It's just a little bit less work.
But the beauty of Linux is that we can make these choices willfully :)
Certainly anyone managing 100 corporate desktops will be deploying a customized image to those machines, rather than using a distro's default installer on each one. In which case, it still doesn't matter what the OS defaults are.
This is the kind of thing that should be automated anyway. Write a script to do everything you're doing in the process of setting up your system, and you won't ever have to do it by hand again.
Because if you have to do it more than once, or, say onto 100 desktops in a work environment, the time spent searching for a distro will pay off. Besides that, some people want stuff to work the first time - and every subsequent time - without having to worry "oh I need to install this on another computer, but I can't remember all the steps I took" in order to get to real actual work.